1. Field of the Invention
The present invention relates to a method and associated device that create a device link profile by synthesizing multiple device profiles in order to carry out color matching among multiple devices via a color management system, as well as to an image processing apparatus in which this method is used. The present invention can be used as a color management system for a digital copier, multi-function peripheral (MFP), printer, monitor or other type of image processing apparatus.
2. Description of the Related Art
In general, peripheral devices such as scanners, digital cameras, printers, and displays have mutually different color characteristics depending on the manufacturer and model. In order to obtain consistent color reproduction independently of these various color characteristics while using these peripheral devices, a color management system (CMS) has been proposed and applied.
FIG. 20 shows the flow of data under a conventional color management system CMSj.
In the color management system CMSj shown in FIG. 20, color conversion, brightness characteristic conversion and other various conversions and adjustments are made to color data CD1 output from the input device DV1, and the resulting data is output to the output device DV2 as color data CD2.
For the input device DV1, a scanner, digital camera, display or the like is used, for example. The color data (image data) CD1 handled by the input device DV1 normally comprises RGB color system-based data (RGB color data) that expresses colors using the three primary colors of R (red), G (green) and B (blue). The color data CD1 output by the input device DV1 is dependent on (or subordinate to) the color characteristics of the input device DV1.
A printer, for example, is used as the output device DV2. The color data (image data) CD2 handled by the output device DV2 is normally CMYK color system-based data (CMYK color data) that expresses colors using the four colors of C (cyan), M (magenta), Y (yellow) and K (black), i.e., the three primary colors plus black. The color data CD2 output to the output device DV2 must be created such that it reflects the color characteristics of the output device DV2, and in this respect, the color data CD2 is dependent on the output device DV2 as well.
Because the color systems that express the color data CD1 and CD2 respectively handled by the input device DV1 and output device DV2 are different as described above, and each of the two respective sets of color data CD is dependent on the color characteristics of its associated device, i.e., is device-dependent, in the color management system CMSj, the color data is subjected to color conversion from one color system to the other color system and the differences in the color characteristics are adjusted.
In other words, in the color management system CMSj, a color system such as the XYZ color system or the L*a*b* color system is used as a color system that is independent from either device, i.e., is device-independent. In order to perform conversion from color data CD that is dependent on the respective devices to color data CE that is expressed in a color system independent from either device, a device profile prepared for each device is referred to.
In other words, in order to convert RGB color data CD1 associated with the input device DV1 into XYZ color system-based data (XYZ color data) CE1, an input profile PF1j comprising an input device profile is consulted. Similarly, for the output device DV2, in order to convert L*a*b* color system (hereinafter ‘Lab color system’)-based color data (Lab color data) CE2 into CMYK color data CD2, an output profile PF2j comprising an output device profile is consulted. For these device profiles, profiles prepared in accordance with the standards established by the ICC (International Color Consortium) are normally used (ICC profiles).
The input profile PF1j comprises a one-dimensional lookup table LUT1 and a matrix MX1, for example. The one-dimensional lookup table LUT1 is used to correct the brightness characteristic of the input device DV1. The matrix MX1 comprises calculation formulae or coefficients for calculation used to convert the corrected color data CD1 into XYZ color data CE1.
The output profile PF2j comprises a one-dimensional lookup table LUT2, a three-dimensional lookup table LUT3 and a one-dimensional lookup table LUT4, for example. The one-dimensional lookup tables LUT2 and LUT4 are used for adjustment of such things as variations in brightness and image contrast in the output device DV2. The three-dimensional lookup table LUT3 is a lookup table used for converting Lab color data CE2 into CMYK color data CD2.
The XYZ color data CE1 is converted into the Lab color data CE2 in a color conversion unit CC1j. In addition, gamut mapping GM is carried out in the color conversion unit CC1j either before or after color conversion XL. During gamut mapping GM, mapping according to a prescribed conversion method is performed such that the color data CD2 stays within a range (color range) that can be reproduced by the output device DV2. For this gamut mapping GM, four methods, i.e., a perceptual method, a method in which a relative color range is maintained, a method in which the absolute color range is maintained and a method that maintains the saturation are known.
The color conversion and the like performed under the color management system CMSj described above is in actuality carried out by a color management module. This color management module (CMM) performs color matching between the input device DV1 and the output device DV2 via the device-independent color spaces using the input profile PF1j and the output profile PF2j. For this color matching, the color management module holds the color conversion steps that include the use of the input profile PF1j, color conversion unit CC1j, and output profile PF2j in a single device link profile in advance, and directly converts the input color data CD1 into the color data CD2 using the device link profile (see Japanese Laid-Open Patent Application 2001-78047).
For such device link profiles, the ICC profiles referred to above are generally used.
FIG. 21 shows the construction of a link profile LP1j comprising a commonly used device link profile, and FIG. 22 shows the functions of the multidimensional lookup table LLU2j in the link profile LP1j. 
With reference to FIG. 21, the link profile LP1j consists of three tables, i.e., an input one-dimensional lookup table LLU1j, a multidimensional lookup table LLU2j and an output one-dimensional lookup table LLU3j. The input one-dimensional lookup table LLU1j is identical to the one-dimensional lookup table LUT1 shown in FIG. 20, and the output one-dimensional lookup table LLU3j is identical to the one-dimensional lookup table LUT4 shown in FIG. 20. The multidimensional lookup table LLU2j is a table incorporating the functions of the color management system CMSj shown in FIG. 20 and excluding the two one-dimensional lookup tables LUT1 and LUT4, as shown in FIG. 22.
As described above, the link profile LP1j consists of three tables into which the color conversion steps are divided. The multidimensional lookup table LLU2j, which is used for the main processing, includes the functions to perform color conversion XL from the XYZ color system to the Lab color system.
Incidentally, while the geometric coordinate distance in the xy chromaticity diagram in the XYZ color system does not match human visual perception, the Lab color system is an uniform color space (UCS) in which the coordinate distance in the chromaticity diagram matches human visual perception. Therefore, color conversion XL from the XYZ color system to the Lab color system includes conversion to match the perceived brightness characteristic.
Specifically, conversion from XYZ values to Lab values is expressed by the following mathematical formulae.L=116(Y)⅓−16a=500(X1/3−Y1/3)b=200(Y1/3−Z1/3)
In other words, in the Lab color system, the brightness characteristic or tone changes as a function that is expressed as the cubic root of the XYZ color system. Therefore, when a multidimensional lookup table LLU2j that includes such color conversion XL is created, the input value and the output value of the table increase or decrease at different rates. For example, where the input value increases at an uniform rate, the output value increases at a diminishing rate as the input value increases. Conversely, where the output value increases at a uniform rate, the input value increases at an increasingly high rate as the output value increases.
Consequently, the problem arises that when the values for the color data to be output are sought via interpolation based on the color data values that are actually input, sufficient accuracy is not obtained.